1. Field of the Invention
This invention is a method of and apparatus for stretching polymeric film in which a web of film is heated in a tenter frame by an air duct/multiple heater combination positioned closely adjacent the web whereby improved preferential heating in a selected lane is brought about to obtain improved film properties.
2. Description of the Prior Art
Methods of selectively, and preferentially, heating polymeric film prior to stretching are known to the art.
U.S. Pat. No. 3,161,711 to Tassler is exemplary of the known art wherein a series of radiant heaters is suitably mounted over the film immediately ahead of the stretching device, such as a tenter frame. Such heaters are disposed, preferably, for the full width of the film web and are spaced apart, parallel to each other in the direction of web travel. The heaters may be individually controlled preferentially to heat the web while it is substantially free from tension along at least one selected longitudinal lane.
U.S. Pat. No. 3,649,726 to Knowles shows a method of controlling the thickness of the polymer film structures in which heaters of the type shown in U.S. Pat. No. 3,161,711 are used preferentially to heat a web of material prior to stretching. Briefly, such method includes measuring the thickness of the film at a location across the width before stretching; measuring the thickness of the film at any location across the width thereof after stretching; computing the cumulative mass values of the film structure across the width thereof before and after stretching; and adjusting the thickness of the film at any point across the width thereof before stretching responsive to deviation from a preselected value in the thickness of the film after stretching corresponding to the cumulative mass value of the film before and after stretching. Again, as with the Tassler patent preferential heating method, the effectiveness of the heating depends upon the effectiveness of the heater used.
Other patents showing other than radiant means for heating a web of film prior to stretching include U.S. Pat. No. 2,339,451 to Bailey et al which shows heating during transverse direction stretching by a plurality of gas burners. The burners are positioned in a zigzag configuration to prevent overheating of individual lanes.
Further, U.S. Pat. No. 2,412,187 to Wiley et al shows the controlling of heat input from internally steam heated radiators to separate portions of the web during machine direction, and prior to cross-machine direction stretching and U.S. Pat. No. 3,304,205 to Ballard et al shows heating alternate edges of the web in a tenter preheat zone to control the transverse direction stretching pattern. The heating means disclosed are radiant or air.
A problem with the teachings of this art, and the other known art, is that it does not show a way to effectively control within very narrow limits the heating of a web in a tenter frame prior to stretching primarily because of the distance of the web from the radiant heater or by failure effectively to isolate the heated air that impinges on the web.
There have been numerous other attempts to improve thickness properties in extruded films; apparatus and methods, for example, have evolved to sense thickness deviations and effect adjustment of film thickness at specific locations usually at or near the point of extrusion of the polymeric material in the film forming operation.
For instance, U.S. Pat. No. 3,122,782 describes a mechanical arrangement for adjusting the thickness of selected hypothetical lanes across a film structure corresponding to thicker or thinner portions of the film. In such patent, two interconnected multiple-junction terminal boards are disposed intermediate a film thickness sensing guage and an adjustment means for changing the width of the extrusion orifice from whence the film structure issues. The film thickness gauge measures the thickness of the film structure and transmits a bias signal through suitable relay elements for energizing motors which are so constructed and arranged to cause opening or closing of the orifice opening in the extruder in accordance with the deviation in the thickness of the film that is sensed by the film thickness measuring gauge.
In U.S. Pat. No. 3,455,175, an extrusion apparatus is disclosed with adjustable lip members containing a plurality of adjustment screws along the orifice slot. The screws are arranged closely together on about 20 mm (0.787 inch) spacing and are threaded into the lip members which have been modified near the extremities by locally weakened cross sections. Manual rotation of the small diameter screws results in displacement of the lip members and by that means the slot opening defining the thickness of the web is varied. Confining the deflection of a lip member to a narrow area approximating the width of a single adjustment screw is most difficult due to the inherent interaction of adjacent screws. Each time the setting of a screw is changed, there is a corresponding unavoidable disturbance of the setting of neighboring screws resulting in an unpredictable deflection of the lip members. To counteract the undesirable deflection, the settings of the neighboring screws must also be corrected. As a consequence, narrow band or streak non-uniformities cannot easily be eliminated without upsetting the basic gauge setting of the lips. As is apparent, adjustment means of this sort require constant personal attention and trial and error manipulation by skilled operating personnel.
Conversely, the difficulties of reliance on skilled operators and trail and error control are to some extent obviated by an automatic gauge control system, such as is disclosed in U.S. Pat. No. 3,122,784, which features a plurality of speed controlled adjustor motors mechanically coupled to displaceable lip segments on a web extrusion apparatus. The motors respond to signals generated by a traversing thickness sensing means located at some distance below the extrusion apparatus which measures the final thickness of the web from standard reference deviations. Electrical signals are thus generated, translated into thickness corrections and distributed by way of switching means sequentially to the input of the motors. This system cannot, however, eliminate narrow streak and gauge band non-uniformities because the individual adjustment means control relatively wide segments of the orifice slot. Moreover, the nature of the nonuniformities is often too subtle and complex for corrective adjustment by mechanical displacement of die lip members.
Another method of improving the thickness uniformity of film is shown in U.S. Pat. No. 3,819,775 wherein means are provided for selectively controlling (by heating or cooling) the temperature of isolated parts of die lips of an extrusion die whereby to improve thickness uniformity. There is no web heating immediately prior to stretching.
And, lastly, U.S. Pat. No. 3,341,388 discloses a method of and apparatus for extruding thermoplastic film wherein heaters are positioned in the die body whereby to heat those areas of the die to help maintain the thickness of the sheet substantially constant.
While these patents and others show polymeric film thickness-controlling or related devices, they do not show controlling such thickness of a web of film, in an improved fashion, by positively controlling the heating in a tenter frame of a selected web lane using air plus a heater positioned in a thermally isolated zone to assure that only a preselected area of the film is preferentially heated as in the instant invention.
The accidental, or incidental, heating of lanes adjacent to the lane selected to be heated is a real problem here. This invention resolves that problem by substantially preventing the escape of heat into adjacent lanes using a duct/heater apparatus of the invention. By so controlling heating, it is possible to induce a substantial temperature rise in a given web lane while raising the temperature of adjoining lanes a minimal amount. In so doing, improved film properties result.